What is the profile plugin?

From Joomla! Documentation

The profile plugin makes it possible to add extra fields to the user profile form in 4 places:

The registration form

The frontend profile editing view

The backend profile editing view (My Profile)

The backend user manager user edit view.

Each of these are controlled independently. They can all be the same or specific fields can be available in each form. Similarly, fields can be set as required independently for each form or all can be the same.

Any field type from the form library can be used.

Optionally the profile for a linked user can be displayed in linked content record.